Sanjeev Latchireddi, Chief Process Engineer – Grinding Americas, Outokumpu Technology, looks at the shift from ball mills to SAG mills. Nowadays, the more successful plants are those that have adopted effective and efficient strategies to optimize their plant operation. As the ‘heart of the plant’ could be viewed as the milling/grinding area, one of the key steps in plant optimization is ensuring the mill is operating properly. In the past, when primary, secondary and tertiary crushers fed material directly to large ball mills, the energy efficiency of the concentrator was determined for the most part by the ball mill operation, whereas now the energy efficiency of a plant often rests largely on the SAG mill operation. As a result, mines have shifted their emphasis in optimization from ball mills to SAG mills.
The rock load in the mill essentially depends on ore characteristics and the discharge rate of broken particles through the discharge end. The discharge rate depends on how efficiently the discharge pump (grate and pulp lifters) operates. Similar to the impeller design affecting pump capacity, the pulp lifter design affects the discharge capacity (or mill throughput) of AG and SAG grinding mills. Generally, the discharge from AG/SAG mills consists of one or both of the following components: slurry (water and finer particles) and pebbles (20-100 mm).
Single stage AG/SAG mills have to handle large amounts of slurry as they are generally in closed circuit with classifiers whose circulating loads reach as high as 400-500%. The geometry of radial and curved pulp lifters is such that the slurry, once passed through the grate into the pulp lifter, will always be in contact with the grate until it is completely discharged, which makes the ‘flow-back’ process inevitable. Slurry ‘carry-over’ is another problem which generally occurs at relatively higher mill speeds and/or with increasing slurry viscosity. Though the impact of flow-back may be of lower magnitude in open circuit grinding, flow-back can make a significant impact when the mills are operated in closed circuit, especially with cyclones and fine screens. The field of breakage diminishes when excessive slurry is present in the mill. The inherent flow-back and carry-over problem associated with radial and curved pulp lifters leads to formation of a slurry pool, which absorbs a significant amount of impact energy instead of being used to cause impact breakage of particles. This inefficient usage of grinding energy reduces the grinding capacity.
In multi-stage ABC/SABC circuits, the AG or SAG mills are in closed circuit with screens and pebble crushers. The mill discharge from these mills consists of slurry, which goes to the ball mills for further grinding, and coarse pebbles/rocks, which are crushed and sent back to the mill. To maximize the capacity of these circuits, the general practice is to use grates with pebble ports (reaching 100 mm) instead of normal grate openings to increase the pebble removal. In addition, operating mills at relatively higher speeds has become an option to increase mill capacity. The reasoning is because the higher the mill speed, the higher the number of impacts/collisions, which in turn is proportional to higher breakage of particles.
With the advent of simulation techniques such as discrete element modelling (DEM), appropriate shell lifters can be designed to operate mills at higher speeds. Shell lifters, an integral part of all grinding mills, are located in the main grinding chamber. However, the inefficiency of pulp lifters increases with mill speed. As a result:
- Pebbles carry-over
In ABC and SABC circuits, once the slurry and pebbles pass through the grate into the pulp lifters, the motion or flow behaviour of solids will be different to the slurry. At the end of one revolution, all the pebbles are supposed to reach the discharge trunnion. However, a significant amount of pebbles are retained inside the pulp lifters.
By the time a pulp lifter starts a new cycle from ‘6 o’clock’, all the pebbles reach the bottom of the pulp lifter and occupy significant volume. The presence of these pebbles blocks the outer rows of grate slots and reduces the flow gradient across the grate. In order to maintain the same flow gradient, the load inside the mill increases and the mill draws more power. In turn, this leads to a higher rock to ball ratio, resulting in insufficient grinding energy and a further increase in load inside the mill.
- Pebbles flow-back
Similar to slurry flow-back, the pebbles flowing back into the mill increase with larger pebble port or grate slot size. As the pebbles flow down and slide across the grate slots, they get an equal chance to go back into the mill. Similar to slurry pool formation, pebbles flow-back would increase the quantity of critical size material in the mill. The amount of pebbles passing through the grate increases with the angle of the grate.
It is imperative to ensure the efficient removal of both slurry and coarse pebbles (critical size) in order to ensure the efficient operation of AG/SAG mills. Elimination of the above mentioned material transport problems will allow the mill to respond truly in terms of power draw for changes in mill load which depends on feed ore characteristics.
Although curved pulp lifters partially solve the ‘carry-over’ problem, they cannot eliminate the ‘flow-back’ problem. This is because once the slurry/pebbles flow into the pulp lifter, they are always in contact with the grate until they are completely discharged out of the mill. However, the curved design necessitates redrilling of the mill head and also requires curved grates, which can be quite complicated compared to a simple radial design.
The best way to eliminate material problems is to use a grate, peripheral discharge trunnion supported mill or use a grate, open-ended discharge shell supported mill. Both of these do not require pulp lifters. The grate, peripheral discharge trunnion supported mill and the grate, open-ended discharge shell supported mill both have inherent structural limitations which makes their commercial application limited. The commercially viable and industry accepted alternate is a grate discharge trunnion supported mill or a grate discharge shell supported mill using pulp lifters. The only way to eliminate the problem of flow-back and carry-over of slurry and pebbles is by optimizing the design of the discharge arrangement.
Outokumpu Technology’s patented new design – TPL™, Turbo Pulp Lifter (patent pending), is a result of these material transport problems. As the internal design of the TPL approximates a grate, peripheral discharge or a grate, open-ended discharge, it keeps the slurry/pebbles away from the grate once they enter the pulp lifter chamber, thus completely eliminating flow-back and carry-over problems.
This efficient material transport ensures the best grinding conditions by allowing the particles to stay inside the mill long enough to be broken into sizes smaller than the grate. The TPL does not necessitate redrilling of the mill head for retrofitting. From the outside, the TPL appears exactly like the conventional radial pulp lifter. Elimination of material transport problems using TPL will bring the following process benefits:
• Allow the mill to operate at maximum capacity
• Ensure good grinding conditions with lower grinding energy per tonne
• Efficient operation even at higher mill speeds
• Operator-friendly smooth mill operation
• Significantly improve wear life
• Can be precisely designed to handle the given capacity
• Can be easily retro-fitted to existing mills.
The optimal performance of AG/SAG mills is the key to successful plant operation. All AG/SAG mills using radial or curved pulp lifters suffer from inherent material transport problems such as slurry and pebble pooling, which decrease throughput and increase energy consumption. The TPL completely eliminates slurry pooling and pebble pooling problems and ensures the best grinding conditions, thus allowing AG/SAG mills to operate at maximum possible capacity with lower energy consumption.